Affairs

My husband and I are best friends with this couple. The wife is my best friend and her husband is my husbands best friend. She told me that she has been having an affair on her husband and I immediately told her to end it! I am so against that! I have not told my husband because I know he would tell her husband ASAP because he is his best friend. Should I tell my husband and her husband ? Or just continue to tell her to stop having an affiar? I really want to tell but I don't feel it's my place to get involved in someone's marriage but I also don't want to hurt her husband either. What should I do ?